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Howard -

Welcome to WC.

I can tell you from experience that a 13' with a 4 stroke 25 hp motor will not perform very well with 3 adults due to the low hp and low torque power band. The right prop will minimize this and the incorrect prop will exacerbate it.

Selecting the appropriate prop is a bit of science and nuance. For the science part we need to know the gear ratio of the motor and what the max rpm range is.

What prop do have on the motor now (brand, size and material) and how does it perform?

At what height is the engine mounted?